In wireless transmission systems, signals circulate between transmitters and receivers through channels. Due to many factors in the channel characteristics, a distortion is induced in the signal transmitted by a transmitter. Therefore, it is generally necessary to determine the characteristics of the channel, at a given moment, in order to estimate the induced distortion of the transmitted signal.
There are a number of techniques for performing channel estimation in wireless transmission systems. One such technique includes transmitting by a transmitter signals with predetermined sequences and comparing the signals received in a receiver using auto-correlation and cross-correlation with the expected signals in order to estimate the characteristics of the channel. The sequences of the transmitted signals are known to the receiver. The results of the correlation operation constitute the estimation of the impulse response of the channel.
For efficient channel estimation, sequences with good autocorrelation properties, such as complementary sequences, e.g., Golay complementary sequences, are transmitted by the transmitter and auto-correlated by the receiver. The property of Golay complementary sequences is their perfect sum of autocorrelations. Another property of the Golay complementary sequences is that the corresponding correlator has a very efficient implementation, which requires only log2(N) additions for two complementary sequences of length N. By comparison, other sequences require N additions to implement such a correlator.
As shown in FIG. 1, in a transmitter 100, a Golay generator 101 generates Golay complementary sequences (Gu, Gv) which are later modulated and transmitted using a modulator 102. The modulator 102 may be, for example, an OFDM modulator, a single carrier modulator, and the like. The Golay generator 101 generates the complementary sequences at sampling rate Ft.
The signals S′ including the sequences G′u,G′v are received at a receiver 110 which operates at a sampling rate Fs which is higher than the rate Ft. It should be noted that due to the channel conditions, the received sequences Gu′, Gv′ may be different than the original sequences Gu, Gv. The received signals S′ are sampled at a rate Fs. However, a Golay correlator 111 should correlate the received sequences at a rate Ft. With this aim, the signals S′ (including sequences G′u,G′v) are filtered using a filter 112, which may be a polyphase filter, to change their sampling rate to a rate Ft of the Golay correlator 111. Then, the cross-correlation results which indicate the channel estimation (CE) as provided by the Golay correlator 111 are up-sampled to a rate Fs by a filter 113. Then, an equalizer (EQ) 114 equalizes the received signals S′ based on the output of the Golay correlator 111. The equalized signals are de-modulated using a demodulator 115. The equalizer 114 and demodulator 115 operate at a sampling rate of Fs.
In millimeter wave wireless transmission systems, the rate Ft utilized by the transmitter and Golay correlator 111 is different from the sampling rate Fs of the receiver. For example, in a millimeter w.ave communication system operable in the 60 GHz band and defined, for example, in the IEEE 802.11 ad standard, the Golay sequence generator rate (Ft=1.76 GHz) is 1.5 times the sampling rate of the receiver (Fs=2.64 GHz), i.e., Fs=Ft*1.5. The oversampling is performed in OFDM and non-OFDM modulated signals in order to avoid sensitivity to timing errors or shaping filters of the transmitter.
As mentioned above one technique for channel estimation at different rates includes down-sampling the received signals to the correlator rate Ft from the rate Fs using a polyphase filter (e.g., a filter 112), applying the Golay correlator, and then up-sampling the correlator's output back to the rate Fs from the rate Ft.
Other techniques include re-sampling the received Golay sequences to the rate Fs, then using a single re-sampled Golay correlator in the rate Fs. However, such a re-sampled Golay correlator does not lend itself to a log2(N) type of implementation as additional N add operators and multiplication operators are required.
In addition to the obvious disadvantages of the prior art solutions in terms of complexity, such solutions induce additional noise and distortions due to the non-perfect re-sampling, which usually involves a finite re-sampling filter.
It would be therefore advantageous to provide a solution that would limit the drawbacks of existing solutions for performing channel estimation using Golay correlators, when different sampling rates are implemented in the wireless transmission systems.